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Kidneys
How does the kidney produce dilute urine?
answer
Cl- is actively transported from the ascending loop.
Na+ is actively transported from the distal tubule.
In the
absence of ADH
, the ascending loop, distal tubule, and collecting
ducts are impermeable to water, and so water does not
follow the Na+ and Cl- ions. The result is dilute urine.
How does the kidney produce concentrated urine?
answer
Cl- is actively transported from the ascending loop into
the renal medulla (Na+ follows passively). The counter-current system of the loop
and the vasa recta blood vessels allows the build up of
high osmotic pressures. In the
presence of ADH
, the
collecting ducts are permeable to water, and so water
is reabsorbed by means of osmosis. The result is concentrated
urine.
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